/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You under the Apache License, Version 2.0
 * (the "License"); you may not use this file except in compliance with
 * the License.  You may obtain a copy of the License at
 *
 *    http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 *
 */

package org.apache.streampipes.processors.changedetection.jvm.welford;

public class WelfordAggregate {
  private Integer count;
  private Double mean;
  private Double m2;

  public WelfordAggregate() {
    count = 0;
    mean = 0.0;
    m2 = 0.0;
  }

  public void update(Double newValue) {
    count++;
    Double delta = mean != null ? newValue - mean : 0.0;
    mean += delta / count;
    Double delta2 = newValue - mean;
    m2 += delta * delta2;
  }

  public Double getMean() {
    return mean;
  }

  public Double getPopulationVariance() {
    return m2 / count;
  }

  public Double getSampleVariance() {
    return m2 / (count - 1);
  }

  public Double getSampleStd() {
    return Math.sqrt(getSampleVariance());
  }
}
